The present invention will be described in connection with a high speed, multiple cavity mold used to mold very thin parts and using high pressures, e.g. up to 30,000 psi, to force the molten plastic into the very thin plastic cavity portions located at a considerable distance from a plastic injection port. The embodiment of the invention illustrated is that of a sixteen cavity mold used to manufacture thin dairy containers for holding cottage cheese or the like. Typically, such dairy containers were run on a 500 ton molding press with a ten or eleven second cycle time for the press. When trying to use higher pressures in order to reduce the cycle time, the higher pressures caused a problem with "flashing" or "flash" as will be defined below. The extra pressure tended to separate mold sections sufficiently at the parting line that flashing became a problem. The present invention is not limited to this particular kind of molding press as it has utility in a large number of plastic molding apparatus and methods where flashing is a problem particularly due to the pressure separation along the parting line rather than due to excessive wear at a portion of the parting line adjacent to the cavity.
The high pressure within the mold cavity exerts a separating force on the mold portions which forces the mold portions away from one another. This separating force is often sufficient to overcome the forces holding the mold portions together, resulting in a slight vertical separation of the mold portions along their horizontal parting line. Upon sufficient separation of the mold portions at the parting line, molten plastic squirts from the mold cavity into the narrow region of separation at the horizontal parting line between the mold portions. This thin extra plastic on the molded part at the location of the parting line is referred to as "flash" or "flashing" and is usually in the shape of an undesirable thin film of plastic extending from the molded part at the parting line. This is undesirable in that the flashing gives the molded parts an undesirable appearance and may interfere with the part's performance. For example, flashing is undesirable in the manufacture of mating parts, such as a container having a complementary lid, wherein flashing at the rim of the container may prevent the lid from fitting on the container properly. Therefore, there is a need for a mold which will preclude flashing at the parting line despite being subject to the higher pressures currently utilized in modern injection molds to obtain high production rates and good surfaces on the molded product.
In order to prevent overheating of a part during molding, channels or vents having a small cross-sectional area are connected to the mold cavity to allow entrapped air to be vented from the cavity. In the press described above, the characteristics of the press and plastic channel or groove are such that a vent span of less than approximately 1 mil in cross section is sufficiently narrow to preclude molten plastic flow therethrough while still allowing for the escape of entrapped air from within the cavity, although the precise opening or space at which plastic flow will begin to occur will vary depending upon the characteristics of the mold, press and the plastic utilized.
In conventional molds, a horizontal or vertical venting groove is provided at a horizontal or vertical parting line, along which the mold portions abut, to allow venting of air from the mold cavity. Upon slight separation of the mold portions caused by the high pressure within the mold cavity, as discussed above, the cross section of the venting groove increases in direct proportion to the increase in separation distance between the mold portions. When the span of the venting groove is increased beyond a critical maximum distance due to mold separation, undesirable plastic flow into the venting groove occurs in addition to the desirable flow of entrapped air therethrough. Molten plastic flow into the venting groove is undesirable in that it results in blockage of the venting from the cavity, thereby preventing escape of entrapped air which causes overheating of parts and also results in undesirable flashing at the parting line, requiring additional machining expense to remove the flashing. Therefore, there is a need for an injection mold which provides venting while still precluding flashing at the parting line upon slight separation of adjacent mold portions.